The invention relates to a process for molding a groove pattern in a rubber tread, the walls of which are joined by at least one rubber connecting element, as well as a molding element equipping a mold for fabricating new or renovated tire treads, in order to use such a process. The invention also concerns tire treads made according to that process.
The term "groove" is understood to mean any recess made in a rubber tread and defined by at least two main rubber walls facing each other, said walls being separated by an average distance representing the width of the groove, the intersection of each of said walls with the rolling surface forming a rubber edge.
"Connecting element" is understood to mean a rubber part forming, when new, a bridge permanently joining the main walls of a groove, so as to maintain sufficient rigidity on the tread provided with said groove.
The published French patent application FR 2,759,323, describes a sculpture of a tire tread containing a plurality of grooves having at least one connecting element mechanically joining two main walls. In that way, it is possible to have a large number of grooves on the entire tread and, consequently, a large number of rubber edges ensuring good performance on a wet road, without that resulting in too great a reduction of stiffness of the tread.
In said patent application, a process is also described for making such a groove, according to which an insert is placed in the unvulcanized rubber tread, provided with one or more openings intended to let the rubber through during the molding and vulcanization of said tread, in order to form the connecting element or elements. Said insert is made of a material capable of withstanding molding stresses and further having the property of being progressively eliminated during rolling of the tire. In the present case, the groove thus formed is almost totally filled with the insert, which reduces its drainage and reservoir capacity accordingly on rolling over a wet road.
It is, of course, possible to remove the insert partially or totally after molding and vulcanization of said tread, but that then adds an operation to the production cycle.
In a second published French patent application FR 2,759,321, another process is proposed for obtaining a tire tread provided with a sculpture containing grooves having at least one connecting element. Said process consists of using a mold formed by two parts, the first part molding the outer surface of the tread corresponding to its rolling surface and the second part molding the inner surface of said tread, both of said parts defining in molding position a volume corresponding to that of said tread. In addition, each part of the mold is provided with at least one molding element projecting on its molding surface and intended to come partially in contact with a molding element borne by the other mold part, in order to mold a groove totally crossing the tread in its thickness. The partial contact between the molding elements in contact forms at least one opening for molding at least one rubber bridge mechanically joining the walls of the groove thus molded.
This second process, making possible easy stripping of the tread after molding and vulcanization, requires the connecting elements, however, to be situated in zones of contact between the molding elements of the first part of the mold and the molding elements of the second part.
FIG. 1, partially showing two rubber blocks 2 and 3 separated by a slit 1 (that is, a narrow groove) and joined by four connecting elements 6, 7, 8, 9, illustrates one particular example of the arrangement of said connecting elements which it is not possible to carry out according to the process described in the French patent application published under number FR 2,759,321 for the simple reason that the elements are situated in pairs above each other in the direction of thickness of the tread, that is, generally perpendicular to the rolling surface of said tread. Furthermore, this process does not make possible the direct production of a tire provided with a tread, but systematically requires an additional stage in the manufacture of a tire. In fact, after fabrication of a tread in a mold in two parts, it is then necessary to add and glue said premolded tread on a tire blank before molding and final vulcanization of said tire in the case of manufacture of a new tire.
All of the other known processes employing a molding element (in particular, in the form of a thin strip) do not make possible the formation of a groove having at least one connecting element whose intersections with the walls of said grooves present contours, all points of which are situated at a distance strictly greater than zero in relation to the bottom of the slit, so that the continuity of volume of the groove is preserved. In fact, every connecting element formed during molding and presenting such characteristics is broken during stripping, the molding element splitting said rubber bridge during that phase.